【作者】 王文博；

【导师】 何晓舟；

【作者基本信息】 哈尔滨工业大学 ， 动力工程， 2019， 硕士

【摘要】 粗糙表面上的热对流是一种常见的情况,在自然界与工业界广泛存在。例如城市表面与大气层换热,海底河床与海水换热,这些都是粗糙表面下的换热。Rayleigh-Bénard对流(Rayleigh-Bérnard Convection,RBC)系统则是从复杂的自然对流现象中抽象出的一种经典流体力学模型并作为本文的实验理论模型。表面粗糙化通常是一种增强换热的方法,但在某些情况下,粗糙表面不一定会增强换热,有时甚至还会降低换热,本文将对这一现象进行实验分析。实验中RBC系统的长(L),宽(W),高(H)之比为4:1:4,Pr=6.2,Ra数的范围为7.47?10~8~4.48?10~9。实验中共对四块不同粗糙高度h=0.5mm、h=1mm、h=2mm、h=4mm的粗糙表面进行测量,分析粗糙表面对换热强度的影响,并通过测量温度剖面,了解其微观机理。实验测量得到,粗糙表面在较低的粗糙高度下确实会对传热产生弱化的效果,并且会使Nu数与Ra数出现非标度律关系的情况。影响传热强度的因素有Ra数的大小,粗糙高度与温度边界层厚度的比值的大小。较低粗糙高度的粗糙表面的边界层厚度与其对应Nu数下的光滑表面的边界层厚度基本相同,在相同Ra数下,会使热边界变厚。其温度剖面的形态比较相似,只是在混合区稍有不同。温度脉动情况在边界层区内与光滑表面相同,同样在混合区出现了不同。温度概率密度分布在边界层区时为高斯分布,在混合区为幂指数分布,并发生正向偏移,在中央区内,同样为幂指数分布,不过正向偏移减少,并且在反向区域出现了指数分布的形态。更多还原

【Abstract】 Thermal convection on rough surfaces is a common occurrence and is widespread in nature and industry.For example,the heat exchange between the urban surface and the atmosphere,and the heat exchange between the seabed and the seawater are all under the rough surface.The Rayleigh-Bénard Convection(RBC)system is a classical fluid mechanics model abstracted from complex natural convection phenomena as the experimental theoretical model of this paper.Surface roughness is usually a method of enhancing heat transfer,but in some cases,rough surfaces do not necessarily enhance heat transfer,and sometimes even reduce heat transfer,this paper will analyze this phenomenon experimentally.In the experiment,the ratio of length(L),width(W),and height(H)of the RBC system is 4:1:4,Pr=6.2,and the range of Ra Number is 7.47?10~8~4.48?10~9.In the experiment,four rough surfaces with different roughness heights(h=0.5mm,h=1mm,h=2mm,h=4mm)were measured to analyze the influence of rough surface on heat transfer intensity,and the microscopic mechanism was understood by measuring the temperature profile.ExperimenTal measurements show that the rough surface does have a weakening effect on heat transfer at a lower roughness height,and a non-scaling law relationship between the Nu Number and the Ra number.The factors affecting the heat transfer intensity are the magnitude of the Ra Number,the ratio of the roughness height to the thickness of the temperature boundary layer.The thickness of the boundary layer of the rough surface with lower roughness height is basically the same as the thickness of the boundary layer of the smooth surface with the corresponding Nu Number,thickening the thermal boundary at the same Ra number.The shape of the temperature profile is similar,except that the mixing zone is slightly different.the temperature pulsation is in the boundary layer zone as same as a smooth surface,but also in the mixing zone.The temperature probability density is gaussian in the boundary layer region,exponentially distributed in the mixing region,and positively offset.in the central region,it is also a power exponential distribution,but the forward offset is reduced,and in the opposite direction,the shape of the exponential distribution appears in the area.更多还原